Recent clinical findings reveal a strong correlation between expression of the colony stimulating factor-1 (CSF-1) and the colony stimulating factor-1 receptor (CSF-1R) in breast epithelial cells and infiltrating macrophages and aggressive breast cancer. This proposal will test the hypothesis that activation of CSF-1R, encoded by the proto-oncogene, cfms, plays a deterministic role in breast cancer pathogenesis and in the evolution of an invasive phenotype. Human CSF-1R cDNA will be transfected into breast cancer cell line, MCF- 7, and an immortalized breast epithelial cell line, MTSV1-7. Activation of cellular mediators, induction of integrins and tumorigenicity potential will be examined. To investigate autocrine contributions to malignant transformation, a CSF-1 cDNA will be introduced into CSF-1R breast cell transfectants. To investigate paracrine effects, co-culture will be performed with CSF-1R breast cell transfectants and myelomonocytic cells engineered to secrete CSF-1. A dominant-negative CSF-1R will be introduced into SKBr-3 which expresses endogenous CSF-1R and CSF-1 to selectively remove those contributions mediated by CSF-1R. These cells will also be utilized to investigate cross-talk between CSF- 1R and p185c-erbB2. The C-terminus of CSF-1R has been implicated in negative regulation of proliferation and structure-function studies will be undertaken with a series of truncation and deletion mutant CSF-1R. Finally, pilot in vitro and in vivo experiments will be carried out to identify novel substrates for CSF-1R in myelomonocytic and breast cells.